Unified Optical Scanning Technology

Chapter 4.1 - Scanner Devices and Techniques: Scanner Technology Organization


Figure 4.1 classifies optical scanning technology, arranged and extended beyond its original 1974 construction [Bei2]. Although some inevitable expansion that developed over the last three decades is represented and clarified here, the basic organization is sustained.

The most durable concept is the division of the technology into two principal categories, namely, "high inertia" and "low inertia." The distinction between the two relates primarily to scan function flexibility, that is, between repetitive and agile control. The high-inertia systems maintain high regularity. They resist rapid changes in scan speed and in the locus of the scanned function. The low-inertia systems, however, allow timely control and alteration of a scan trajectory. They often exhibit "random access" capability.

A novel representation in Figure 4.1 illustrates this separation of task with the appearance of "oscillatory" under both the high inertia and low inertia categories. The low-inertia oscillatory descriptor leads to the galvanometric devices, as represented alongside the nonmechanical—Acoustooptic and Electrooptic—devices. (Each of these techniques is described in a later discussion.) The galvanometer scanner continues to be identified as low inertia, for its low moment of inertia* rotor is in a sufficiently damped broadband suspension to support variable

Fig. 4.1 Classification of optical scanning Technology. After "Laser scanning systems," by L. Beiser in Laser Applications, Volume 2, by Monte Ross, ©1974, Elsevier Science (USA), reproduced by permission of the publisher.

velocity and/or variable rate scan cycles. This includes the formation of the important "sawtooth" ramp having a relatively short retrace interval and the ability to "random access" any point in an image format within its retrace time limitation.

In sharp contrast to the above, the high-inertia "rotational' block leads—perhaps surprisingly—to the oscillatory resonant scanner, in the same domain as the fully rotational polygonal and holographic scanners. Although the resonant scanner may appear physically similar to the galvanometer, it forms a sinusoidal scan function. This unique oscillating frequency is not readily adjustable, and its low-loss resonant suspension sustains this frequency, disallowing rapid start or stop. Thus, although the rotor of the resonant scanner may actually exhibit a low moment of inertia (as does the galvanometer), it exhibits the scanning properties of a high-inertia device. Significant further distinctions between the galvanometer and resonant scanners are presented in Section 4.5.2.

Also added to the high inertia category are the translational devices, such as mechanical transports of the object, the storage medium, or the optics. In most cases, these fall into the category of objective scanning, described in Section They need not be linear transports. An excellent example is illustrated in Figure 1.11 of the above-referenced section: a drum supporting a storage medium rotating under a focusing objective lens that is also translated longitudinally, parallel to the drum axis. Capable of forming an extremely precise helical scan, this important 'external drum" system articulates in a manner similar to the machinist lathe—and, notably, similar to the Edison cylinder. Further detailing is reserved for subsequent discussion.

* Moment of inertia ≡ mr2; m = effective mass at r - radius of gyration.



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